Whereas the display of a waveform that follows measured variations in bloodpressure can be interpreted by an expert observer with considerable accuracy, there are many situations where it would be far more desirable to indicate each of the systolic, diastolic blood pressures by a single number. In some instruments, the systolic blood pressure is the highest pressure occurring in a five-second interval, but this gives too high a reading in the presence of severe respiratory variation and the readings vary in a way that appear to correlate poorly with the observed waveform. In more recent monitors, systolic and diastolic detectors are synchronized with the EKG and updated at each heartbeat and are slightly filtered. Because the numbers follow the respiration, it is up to the user to make a mental average in order to estimate the true blood pressure. With the advent of microprocessors, various data filtering techniques have been employed, but they have depended on past history in such a way that if they are sufficient to reduce the effects of certain artifacts by a significant amount, their response to sudden changes in actual pressure is too slow. Furthermore, the systolic, diastolic and mean pressures have been processed separately so as to give variations that do not correspond even though they are related to the same factors. Other monitors require the presence of EKG machines or a plethysmograph separation apparatus.
One of the most difficult problems has been to obtain a reliable pressure number under conditions where the blood pressure is highly variable, such as when the pulmonary artery pressure is being measured, because the effects of inspiration and expiration are very great. In this case, the beat-to-beat variation in mean pressure is often greater than the means pressure itself. It is commonly accepted that the best time to read the pulmonary artery pressure is at the end of expiration when the conditions of small and stable airway pressure are most likely to be met. Whereas it is virtually possible to identify the beats taking place during this time, it is not easy to design a monitor that can isolate them. In one approach to this problem, each of the three pressures is treated independently and the weight given to any peak is generally inversely proportional to its deviation from an exponential average. This can result in a pressure that coincidentally is the same as the average being given for too much weight. This and similar methods are completely ineffective when there is little or no pulsation.